The advancements in one-dimensional semiconductor nanostructures, are attracting significant attention with respect to their potential applications in nanoelectronic and photonic devices. This is due in part to the ever increasing problem of electromigration and subthreshold leakage in the miniaturization of electronic components for modern information industries.
Nanowires offer thermodynamically stable features and are typically defect-free and single-crystalline and thus have a number of advantages over thin films with respect to high-performance one-dimensional arrays of transistors. As some of the most important semiconducting materials, Si and Ge nanowires have also attracted a wide interest due to their unique size dependent electronic and optical properties and have been successfully implemented in high capacity power cells, field effect transistors and third generation solar cells.
The more complex, core-shell nanowires of Ge/SiOx have drawn considerable interest due to the higher intrinsic carrier mobilities of Ge, making it the more suitable material for a carrier channel and using Si as the shell material, which simplifies the chemical passivation of the structure, with SiOx being a much more stable and higher quality surface passivation than Ge oxides. Si and Ge nanowires have been prepared by employing a variety of known different techniques, such as laser ablation, physical thermal evaporation, chemical vapour deposition, and the more popular, vapour-liquid-solid (VLS). To date the simplest way to produce silicon or germanium nanowires has been with the VLS mechanism of growth. Many experimental variations have been used to achieve this including CVD, laser abalation and physical thermal evaporation, all of which require the need of metal catalyst.
The widely used VLS growth mechanism provides direct control of nanowire diameter and length and can be applied to a variety of materials including single and compound semiconductors from the group IV, III-V, and II-VI systems. Variations of VLS have been developed to realize anisotropic growth in solution (SLS) and in supercritical fluids (SFLS). In particular, the supercritical fluid-liquid-solid variation has been demonstrated as a tuneable approach to synthesising large yields of high quality nanowires.
Supercritical fluids are unique in that they can diffuse through solids like a gas while dissolving materials like a liquid and have been successfully used as fluid mediums for monomer diffusion in mesoporous templates. The application of these methods in realizing one-dimensional (1D) nanostructures is considered to be highly promising for scalable, economical, and controllable growth of a variety of elemental and compound 1D nanocrystals.
The following publications relate to the non-seeded synthesis of silicon and germanium nanowires.                (1) Kim, B.-S.; Koo, T.-W.; Lee, J.-H.; Kim, D. S.; Jung, Y. C.; Hwang, S. W.; Choi, B. L.; Lee, E. K.; Kim, J. M.; Whang, D. Nano Lett. 2009, 9 (2), 864-869.        (2) Gerion, D.; Zaitseva, N.; Saw, C.; Casula, M. F.; Fakra, S.; Van Buuren, T.; Galli, G. Nano Lett. 2004, 4 (4), 597-602.        (3) Gerung, H.; Boyle, T. J.; Tribby, L. J.; Bunge, S. D.; Brinker, C. J.; Han, S. M. J. Am. German Chem. Soc. 2006, 128 (15), 5244-5250.        (4) Zaitseva, N.; Dai, Z. R.; Grant, C. D.; Harper, J.; Saw, C. Chem. Mater. 2007, 19 (21), 5174-5178.        (5) Zaitseva, N.; Harper, J.; Gerion, D.; Saw, C. Appl. Phys. Lett. 2005, 86 (5), 1-3.        
These publications attempt to provide a method for the production of pure single crystal Ge nanowires using a non-seeded synthetic route. A problem with the production of pure single crystal Ge nanowires is that the process is technically difficult to achieve.
Si and Ge nanowires synthesis has received intense investigation, given their high compatibility with standard CMOS technology and their expected ease of integration in future electronic devices. To realize group IV nanowire based electronic devices, it is important to gain precise control of their length, diameter, purity and structural quality. Of these, one of the most crucial problems has been nanowire purity due to the contamination occurred through the use of metal seed catalysts in the VLS technique.
The VLS growth mechanism utilizes a metal catalyst seed particle to form a eutectic alloy droplet with the desired semiconductor material. Saturation of the alloy droplet with additional monomer leads to the extrusion of a one dimensional semiconductor nanowire at reduced temperatures afforded by the eutectic alloy. The metal catalyst incorporation is needed given the high stability of Si and Ge compounds and the extremely elevated temperatures required to nucleat group IV crystalline material. In general, Si and Ge nanowires are most commonly grown using the VLS or SFLS processes by employing heavy metals such as Au, Bi, Co, Cu, Ni, and Fe as catalysts.
Gold (Au) is the most widely used catalyst for silicon nanowire growth because of its simple eutectic-type phase diagram and low eutectic temperature (360° C.). However, Au is also known to diffuse very rapidly into silicon and makes deep centers that increase p-n junction leakage and decrease dielectric strength, which is problematic. It has been noted that the metal from the catalyst particle wets the nanowire sidewalls, causing a tapering of the nanowire diameter and eventually consuming the droplet and terminating VLS growth. As a result the nanowires being developed from seeded synthetic routes are not pure enough to warrant electronic device incorporation.
To date, Saw are one of four groups internationally, who have reported the non-seeded synthesis of Ge nanowires, however none of these groups have managed to achieve sufficiently high yields or monodispersity of the nanowire dimensions from their systems.
An object of the invention is to provide a process and apparatus to provide non-metal seeded pure Ge and/or Si semiconductor nanowires to overcome the above mentioned problems.